To begin: Place capers, anchovy, parsley and a pinch of salt into a mortar and pestle and pound to a rough paste. Stir through Dijon mustard and a squeeze of lemon. Set the parsley paste aside.
With a sharp knife carefully butterfly the salmon fillet open and lightly season the cut surface of the salmon. Spread the parsley paste over the cut surface of the salmon and close the salmon back into the original shape and set aside. Cut a sheet of puff pastry in half and place the salmon fillet on the pastry, arrange the cut asparagus on top of the salmon. Place an egg yolk in a mixing bowl and break it up with a pastry brush. Brush the edges of the pastry with the egg yolk and
place the other half of the pastry over the salmon.
Seal the pastry removing any air bubbles. Using the tines of the fork seal all the edges of the pastry and cut away the excess.
Score the top of the pastry and brush the entire surface with the remaining egg yolk.
Place a sheet of baking paper on a solid baking tray and the salmon pastry on top. Place the tray into a pre-heated oven 180 °C for 15 minutes until golden brown.
Once baked remove the tray from the oven, discard the baking paper and rest the pastry on the hot tray for 5 minutes. Once rested, slice the pastry in half and serve with lemon and a crisp green salad.